This invention relates in general to composite structures and, more specifically, to the manufacture of composite structures having a foam core and one or more reinforcing layers of high strength fibers embedded in a thermosetting resin matrix.
Structures in which high strength fibers are embedded in a synthetic resin matrix have come into widespread use due to the high strength to weight ratio of such materials and the general ease of forming complex structures. Originally, fibers such as glass fibers were impregnated with a thermosetting resin such as a polyester and laid up by hand over male molds. While suitable for many products, these methods are labor intensive, do not produce high precision structures and tend to have less than optimum strength and weight, due to bubbles or other voids in the product and uneven resin distribution.
Where higher quality and higher strength is required, higher performance fiber such as graphite and boron fibers, and methods of applying pressure to the shaped structure during resin cure have been developed. For simple structures, hand layup of resin impregnated sheets followed by autoclaving at high temperature and pressure or the application of pressure in a press or with vacuum bag at elevated temperatures often produce excellent products. However, these methods all require complex, expensive and slow equipment, considerable hand work and are not very suitable for complex structures.
For more complex shapes such as channels, wing boxes, missile fins and the like, a variety of complex, multi-component tooling has been developed to apply pressure at appropriate places across a hand laid-up preform during high temperature curing. Attempts have been made to use elastomers having positive coefficients of thermal expansion as the pressure application means, as described by Prunty in U.S. Pat. No. 4,388,263 and Arachi in U.S. Pat. No. 4,167,430. While effective where carefully designed for specific parts, these systems are complex, expensive to design and use and may require frequent replacement of the elastomeric pads which change expansion characteristics after a number of high temperature cycles.
Thus there is a continuing need for improved methods of producing high quality, void-free, uniformly impregnated, composite structures at lower cost.